Bias (Statistical Bias)

Any deviation of results or inferences from the truth, or processes leading to such deviation. Bias can result from one-sided or systematic variations in measurement (systematic error); flaws in study design; etc. There is no sense of prejudice or subjectivity implied.

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Heterogeneity: Subgroups, Meta-Regression, Bias And Bias-Adjustment [Internet]

This Technical Support Document focuses on heterogeneity in relative treatment effects. Heterogeneity indicates the presence of effect-modifiers. A distinction is usually made between true variability in treatment effects due to variation between patient populations or settings, and biases related to the way in which trials were conducted. Variability in relative treatment effects threatens the external validity of trial evidence, and limits the ability to generalise from the results, imperfections in trial conduct represent threats to internal validity. In either case it is emphasised that, although we continue to focus attention on evidence from trials, the study of effect-modifying covariates is in every way a form of observational study, because patients cannot be randomised to covariate values. This document provides guidance on methods for outlier detection, meta-regression and bias adjustment, in pair-wise meta-analysis, indirect comparisons and network meta-analysis, using illustrative examples.

Blinded versus unblinded assessments of risk of bias in studies included in a systematic review

When researchers want to answer a question they can use an approach called a systematic review, which is intended to examine all of the studies that have been done in a particular area of interest. When examining and summarizing the literature, researchers are expected to determine which of the studies were well‐conducted (i.e. high quality) and those that were not. What we do not know enough about is how researchers should conduct the assessments to determine which studies were of high quality. This is important because if the researcher is aware of certain study characteristics (e.g. what journal the study was published in) they may inadvertently assess the study a certain way. For example, if the author of the study is well‐known to the assessor, they may be more likely to assume it is of 'high quality'. Our research examines whether blinding researchers to study characteristics makes a difference when the goal is to summarize the literature. We only found a few studies that reported data relevant to our question. The results from these studies were inconsistent, however, the results suggest that it may not make a difference if quality is appraised under blinded or unblinded conditions during a systematic review.

Publication bias in clinical trials due to statistical significance or direction of trial results

The validity of a systematic review depends on the methods used to conduct the review. If there is a systematic bias, such that studies with statistically significant or positive findings are more likely to be published and included in systematic reviews than trials with non‐significant findings, then the validity of a review's conclusions can be threatened.

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Terms to know

Inference
Inferences are steps in reasoning, moving from premises to conclusions. Statistical inference uses mathematic...
Risk of Study Bias
Bias can result from several sources: flaws in study design; interpretations or analyses based on flawed data...

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